here's my totally impractical, brute force way to catch zodiac
zodiac very likely had a driver's license. he was certainly driving to get around, i gather enforcement of the requirement to have a license was decent by then, and the last thing he would want is to get a ticket with a bloody rag in the trunk 2 blocks from a murder.
IIUC, the state of california kept physical copies of driver's licenses issued in the late 60s and early 70s. IIUC, these included copies of the pictures and the descriptive information (height, weight, eye, and hair color). i gather some of these have been digitized and entered into california's searchable database to some unknown degree, including pictures and biometrics. if most or all of the remainder still exist in boxes, file cabinets, etc., first digitize them all, for some reasonable window of time (1969 - # of years they're good for, through 1971). yes, that's ~ 10 million people.
narrow to people living within 2-3 hours' drive of all the murder scenes. then narrow by gender, race, height range, weight range, based on witness descriptions. that's still hundreds of thousands.
but we have more information we can use to narrow further. testimony of his hair color is imprecise, but it seems fair to say it was neither platinum nor jet black. let's say that shaves off 10%. then we apply machine learning to the pictures to rule people in or out (or inconclusive). he did not have any obvious distinguishing features such as moles, scars, tooth gaps, unibrows, cleft chin, etc. so throw out any people whose pictures show they DID. shaves off another 20-30% or so. witnesses don't describe him as having especially narrow- or wide-set eyes. so throw out outliers for eye-spacing. etc. in this way, i think you could get the suspect pool down to something like 50k people.
next, look up everyone's birth or immigration records. if anyone's born outside the US, toss em. zodiac was american. shaves off another few percent. next, do sleuthing on everyone you can find. throw out anyone who grew up east of the mississippi, or in texas or minnesota. all witnesses agree zodiac had no discernible accent (beyond american). that likely means he grew up somewhere in the american west, outside of texas and minnesota. this might be more of a "soft" classifier that sorts strength of suspects rather than ruling them in or out... next, try to identify military members, and see whether they were deployed at the time of any of the killings. throw out anyone found to have been overseas or in another state at the time. ...
by being sufficiently creative yet careful, i think you could get the pool down to 4 figures or at least low 5 figures. then you ignore due process and fingerprint all of them, and compare the prints to the ones from the stine scene that were in blood.
in this way, the state of california could spend a billion dollars it doesn't have, and grossly violate the constitutional rights of 1000s of people, in a bid that has maybe 50-50 odds of succeeding at identifying one serial killer who's probably already dead.